This invention relates generally to conductive polymers and more particularly to spreadable conductive polymer coatings having positive temperature coefficients (PTC) of resistivity and to application of such coatings in useful articles.
There are currently available conductive polymers with PTC characteristics which are produced by dispersing carbon powder or the like in a melted crystalline polymer such as polyethylene and then molding the mixture to produce the desired article. Since a composition of this kind will not dissolve in solvents, it must be heated above its melting point, mixed, and extrusion molded to form desired shapes. As a result, the electrical resistivity is high, the variety of shapes is limited, and mass production is difficult if not impossible.
Application of such compositions to planar heating elements has produced unsatisfactory results due to the difficulty of fabricating the thin bodies required while maintaining uniform conductivity. Moreover, under service conditions in which the conductor is repeatedly flexed, the conductivity has decreased due to cracking of the bodies on both micro and macro scales.
Attempts to make spreadable conductive polymers by mechanically dispersing fine carbon powders in a resin to make a screen-printable paste or coating have had very limited success. They have provided unacceptably high resistivity, non-uniform resistivity, excessive variation of resistivity with temperature increases due to low bond strength between the resin and the carbon powder, and excessive weakening of the films after exposure to high temperatures. Thus, an attempt was made to prepare a conductive polymer paste by graft polymerizing a high polymer onto the surfaces of fine carbon particles to provide a network structure with improved bond strength between the carbon and the polymer. This resulted in a loss of flexibility when the resistivity was reduced to the desired level.
To the present time, all conductive polymers available have exhibited at least one or more of unsatisfactory PTC values, insufficient stability after repeated thermal cycling, inadequate mechanical strength and flexural endurance, and low fabricability.
The foregoing illustrates limitations known to exist in present conductive polymers. It would be advantageous to provide an alternative directed to overcoming one or more of those limitations. Accordingly, a suitable alternative is provided including features more fully disclosed hereinafter.